Measurement transmitters are frequently used to monitor a process parameter of a medium. Process parameters can be, for example, but not limited to, pressure, fill level, flow rate, pH, temperature. Measurement transmitters of this type have the following features:
a sensor for registering a parameter of a medium, with the sensor issuing a primary signal dependent on the parameter;
a transmitter housing, which defines at least one chamber in its interior, and which has a first opening on the side facing the medium, and a second opening; and
an electronic circuit, which is arranged in the at least one chamber, with the circuit processing the primary signal of the sensor, and producing an output signal, which is issued over a signal path extending through the second opening.
Through the first opening on the side facing the medium, the sensor can register the parameter to be measured, provided that the sensor is arranged in the transmitter housing, or the primary signal can be conducted into the transmitter housing through the first opening on the side facing the medium, if the sensor is arranged outside of the transmitter housing. In the area of the first opening, a seal is normally provided, which protects the at least one chamber from penetration of the medium.
A failure of the seal could, however, open a contamination path reaching from the first opening to the second opening, and extending through the at least one chamber. From the second opening, the medium could then escape into the environment, or even into a control room or measurement room, if piping is provided for the rest of the signal path, for example a pipeline in the sense of a conduit system as specified in NEC 501-4. The American standard ANSI/ISA-12.27.01-2003 with the title “Requirements for Process Sealing Between Electrical Systems and Flammable and Combustible Process Fluids” takes this problem into account. This standard requires that first and second seals be provided, which interrupts the contamination path, and that, in addition, means be provided to monitor the contamination path between the first and second seals in order to indicate a failure of the first seal.